1. Field of the Invention
This invention relates to an improved catalytic process of dewaxing and desulfurization of gas oils.
2. Discussion of Prior Art
Catalytic dewaxing of high-pour gas oils to low pour No. 2 fuel over a shape-selective zeolite catalyst of the ZSM-5 family which selectively cracks long-chain normal paraffins, slightly-branched isoparaffins and long-chain cycloparaffins is known in the art (e.g., U.S. Pat. Nos. 3,700,585 and its reissue, Re. 28,398, the entire contents of both of which are incorporated herein by reference). The catalytic dewaxing process disclosed in these patents (also known as Mobil Distillate Dewaxing or Distillate Dewaxing, MDDW and DDW, respectively), is usually followed by a conventional hydrodesulfurization process (HDS) to remove substantially all sulfur from the product of the catalytic dewaxing process. The conventional hydrodesulfurization process is usually already present in a refinery; thus, the new catalytic dewaxing process is incorporated into the refinery operations upstream of the HDS process.
Prior to the development of the catalytic dewaxing process, high pour point gas oils were dewaxed by a conventional solvent dewaxing treatment. Although solvent dewaxing was also usually followed by the HDS process, it has been suggested in prior art to first subject the sulfur-containing high pour gas oils to the HDS process and then to the conventional solvent dewaxing process (see, e.g., U.S. patents to Murphy et al., U.S. Pat. No. 3,520,796 and to Offutt et al., U.S. Pat. No. 3,617,475). Murphy et al. claim that this sequence of operations reduced pour point of the product, and Offutt et al. that it produces a product with better hazing characteristics. However, in the process sequence of both Offutt et al. and Murphy, Jr. et al., the two unit operations (the HDS and the dewaxer) are used as physically and conceptually separate units connected only by the oil base stock entering the HDS unit and by the desulfurized product of the HDS unit entering the dewaxer. The only advantages claimed by Offutt et al and by Murphy, Jr., et al. relate to the final product qualities.
In contrast, it has now been discovered that substantial process and cost advantages can be attained if the conventional HDS unit is followed by the catalytic distillate dewaxing process unit (hereinafter DDW), and if a number of process streams flow between the units to maximize the utilization of compression and heat exchange capabilites between the two units.